How to calculate New Fans from Total Fans using dates in Google Data Studio

I am pulling Facebook Fan data via Supermetrics into Data Studio and I was hoping someone could share a formula that I could use to calculate New Fans, as a calculated field, from Total Fans. 

The formula would need Identify the last day of the month and the subtract followers from the first day of the month.

For example: If there 100 Fans at the end of September and 60 Fans at the beginning September, the formula would show 40 New Fans.